A new study suggests that an unlikely source is holding on to more carbon than all the plants on land: massive aquifers under the world’s deserts. This kind of “carbon sink” is a phenomenon researchers have been aware of for some time, but new data reveals how much carbon might be captured in the desert, and the report’s authors suggest that this information can help determine the amount of fossil fuels humans can burn before exceeding the planet’s capacity.

The burning of fossil fuels and deforestation are two primary sources of carbon emissions. According to the University Corporation for Atmospheric Research, about 40 percent of that carbon stays in the atmosphere and some 30 percent enters the ocean. Researchers know planets absorb some carbon dioxide, but it’s been clear for a long time they don’t consume all of it, and nobody knew where the rest of the carbon ended up. This new study, conducted by researchers in China, reveals the “missing carbon sink.”

The study shows carbon is swept up by rainwater and irrigation from agriculture, which then sinks into the ground – particularly in dry desert areas. Underground aquifers store the carbon deep below the desert surface, where it will not re-enter the atmosphere. Data collected in the new study projects that the amount of carbon trapped in these “sinks” is 14 times greater than originally understood, and that the massive underground aquifers, when totaled together, cover an area larger than the continent of North America.

Although it’s impossible to determine exactly how much carbon is stored in these sinks, researchers estimate the world’s desert aquifers must be holding about 1 trillion metric tons (1 trillion U.S. tons) of carbon. That’s about 25 percent more than the amount stored in living plants on land. This research is a baby step toward the understanding of how these carbon sinks work and how much they are capable of consuming, but researchers hope that, with future studies and additional information, this knowledge might be useful in determining practical limits for the human contribution to carbon emissions.